Try this.
First make sure you have backups of your library and media (sounds like you do, but just in case). You might want to try this first with a small number of files selected.
Go to a file view and select all media (CTRL+A)
Right-Click=>Library tools=>Rename,Move,Copy Files
In the dialogue box make sure that in the top drop down menu you choose "Update Database to Update to New Location (no file rename, move or copy)
Put in the new path name and check in the preview pane that it looks ok before hitting the OK button.
If files are networked, I would recommend mapping the drive/path before you do this.
PS - you can't delete the default "Main" Library if that's what you are using-- you can clean it out by using File=>Library=>Clear. This is the nuke option unless you really want to start from scratch, Not recommended.